I'd rather have people who are more experienced and calm lead, rather than a person in shock and naive towards the situation in a panicked manner in an uproar. Yeah, we can infer that the other nobles could remove the royal family of power, but what can she do? All the knight guy did was tell the nobles to inform the princess of the situation, but looking at her reactions of whether the brother could wake up or not after the knight just said to inform her, she isn't mentally stable to hear that information yet and not reliable to be a leader. (Or the nobles were dodging the question) We, as a readers, don't think she is special currently, only her family is special. But to say that knight guy is good? I would say he's a sociopath, thinking she could handle the situation when literally after he said to tell her everything, she immediately became worried by a question he asked to himself, even though everybody should be questioning these questions when thinking about the future.